ALTERNATING CURRENT MACHINES – I(Practical:)

6. Perform the polarity test of a single phase transformer.

Perform the polarity test of a single phase transformer.

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5.1 Draw the required circuit diagram.

5.2 Collect tools, equipment & materials required.

5.3 Connect the equipment according to the diagram.

5.4 Connect the circuit to the source.

5.5 Record the voltmeter readings.

5.6 Determine whether it is additive or subtractive polarity.

We do polarity test on parallel transformers to ensure that we connect the same polarity windings and not the opposite ones. If we accidentally connect the opposite polarities of the windings, it will result in a short-circuit and eventually damage the machine.

We can categorise the polarity of the transformer to two types,

1. Additive Polarity
2. Subtractive Polarity   

       

   

   Additive Polarity

   In additive polarity, the voltage (V_c) between the primary side (V_a) and the secondary side (V_b) will be the sum of both high voltage and the low voltage, i.e. we will get V_c = V_a + V_b

   Subtractive Polarity

In subtractive polarity, the voltage (V_c) between the primary side (V_a) and the secondary side (V_b) will be the difference of both high voltage and the low voltage, i.e. we will get V_c = V_a – V_b

In subtractive polarity, if V_c = V_a – V_b, it is a step-down transformer and if V_c = V_b – V_a, it is a step-up transformer.

We use additive polarity for small-scale distribution transformers and subtractive polarity for large-scale transformers.

Procedure of Polarity Test of Transformer

1. Connect the circuit as shown above with a voltmeter (V_a) across primary winding and another voltmeter (V_b) across the secondary winding.
2. If available, take down the ratings of the transformer and the turn ratio.
3. We connect a voltmeter (V_c) between primary and secondary windings.
4. We apply some voltage to the primary side.
5. By checking the value in the voltmeter (V_c), we can find whether it is additive or subtractive polarity.

If additive polarity – V_c should be showing the sum of V_a and V_b.
If subtractive polarity – V_c should be showing the difference between V_a and V_b.
Caution: Be careful that the max. measuring the voltage of voltmeter V_c should be greater than the sum of V_a (Primary winding) and V_b (Secondary winding) otherwise during the additive polarity, the sum of V_a and V_b comes across it.
Note: If we require additive polarity, but we have subtractive polarity, we can simply change it by keeping any of the primary or secondary windings in the same fashion and reversing the winding connection of the other one. Similarly, if we require subtractive polarity but have additive polarity, we could do the same procedure as above